The present invention relates to the control of operating torque in torque or thrust inducing systems and, more particularly, to an electronic torque switch for controlling the operation of electric torque motors.
Electric torque motors, such as AC induction motors, are used in a variety of applications where output torque is transferred via a mechanical transmission or operator and appears as torque or thrust on a workpiece being operated. For example, electric motor driven operators are used extensively for the operation of various kinds of valves in the power generation, chemical and petrochemical industries. Remote operation of these valves is often necessary or desirable because of the critical nature of the valve operation or its hazardous location. Further, precise control of valve operation is often necessary both to maintain proper control of a critical process or to protect personnel or equipment. Also, in most cases a motor operated valve cannot be installed and set up (or inspected and tested after installation) under actual operating conditions with fluid flow in the system in which the valve is installed. Therefore, motor operated valves are typically adjusted for proper operation, either initially or after periodic maintenance, under static conditions. It is well known, however, that many of the characteristics of valve operation change or vary substantially under dynamic operating conditions. Such variations may be immediately apparent or appear or become aggravated with time and are dependent on such conditions as the actual fluid operating pressure, operator gear train and valve packing wear, variations in operating voltage supplied to the motor, and the quality of periodic maintenance.
The torque delivered by a torque motor in a torque or thrust inducing system, such as the torque applied to a valve stem by a motor-driven operator, is typically controlled by electromechanical torque switches. A torque switch in a typical mechanical valve operator assembly operates off of a motor driven worm in the transmission assembly delivering torque to the valve stem. The torque-induced thrust on the valve stem is transmitted to the worm and results in a force tending to move the worm on its longitudinal axis. The worm is typically attached to the operator housing through a spring pack, and as the axial thrust on the worm increases as a result of increasing stem thrust, the spring pack will deflect and the worm will move axially. Axial movement of the worm against the bias of the spring pack is used to move a torque switch arm, the movement of which is adjustable, to set the point at which the torque switch contacts open to stop the torque motor. Motor operated valve assemblies also utilize electromechanical limit switches, typically operated by gear train movement, to stop valve travel in the open direction before the valve backseats, to bypass the open torque switch as the valve is being unseated, to provide remote light indication of valve position, and to actuate interlocks with other equipment. A detailed discussion of the construction and operation of a motor operated valve, controlled by the above type of electromechanical torque and limit switches is contained in U.S. Pat. No. 4,856,327, entitled "Method and Apparatus for Monitoring and Measuring Dynamic Loads in Thrust Inducing Systems".
As indicated in the foregoing patent, torque and limit switches used to control motor operated valves are typically difficult to adjust, require the use of skilled personnel, and are typically rather imprecise such that minor changes in the switch setting can result in large changes in stem thrust actually applied to the valve. In addition, however, an equally serious problem results from the fact that the spring pack displacement utilized to actuate the torque switch is not a direct indication of actual stem thrust and, as a result, the actual thrust levels at which torque switch trip occurs may be either far lower than that required to properly seal the valve or higher than the levels which the valve and valve operator can safely withstand without damage. It is suspected that inconsistencies in the operation of electromechanical torque switches may be attributed to the Belleville spring pack. Although the apparatus and method disclosed in the above identified co-pending application provide a significant improvement in monitoring the operation of motor operated valves utilizing state of the art electromechanical torque switches, ultimate control of torque motor operation still relies on the electromechanical torque switch, the deficiencies of which in controlling the thrust output of a motor operated valve assembly are well documented. Thus, although actual operating thrust values may be measured and monitored accurately, reliance on existing switches to ultimately control torque output results in a number of serious potential problems, such as: (1) unacceptable deviations in repeatability of operation, (2) unacceptably large thrust changes resulting from minor changes in switch settings, (3) grease migration from the operator into the mechanical torque switch assembly affecting proper operation and resulting in unacceptably high thrusts, (4) the need to make torque switch adjustments at the operator itself, requiring highly skilled personnel and the use of extreme care, and (5) the loss of safety margins built into torque switch settings as a result of valve packing adjustments, operator wear, or poor maintenance procedures.
There is, therefore, a need for a reliable torque switch for more accurate control of the torque delivered by a torque motor in a motor operated valve assembly or similar thrust inducing systems. Such a switch should also be compatible with and usable in existing motor operated valve designs, either as a part of an original installation or on a retrofit basis.